1. Field of the Invention
The present invention relates to the manufacture of an aluminum alloy article exhibiting improved bake hardenability. More particularly, the present invention relates to an aluminum alloy product of the Aluminum Association ("AA") 6000 aluminum alloy series exhibiting improved paint bake response when utilized for automotive purposes, such as vehicular panels.
2. Description of the Related Art
Workers in the field commonly employ aluminum alloy sheet products, such as vehicular panels, during the manufacture of automobiles. They prefer aluminum panels because of their light weight. In addition to being light weight, it is important that the vehicular panels have good strength properties as well as exhibiting good formability at room temperature. Thus, for many years, workers in the field made serious attempts to manufacture aluminum alloy products for the automotive industry which satisfy both of these requirements. For example, U.S. Pat. No. 4,784,921, to Hyland et al., describes an aluminum alloy automotive material of the AA 2000 alloy series having improved strength and formability properties. This reference teaches that the improved properties are obtained by solution heat treating at temperatures between 900.degree. and 1100.degree. F. to produce a structure of fine grain size, followed by rapid quenching of the product from the solution heat treatment temperature to 350.degree. F. or lower at a rate of at least 10.degree. F./sec., preferably at least 300.degree. F./sec. Cooling from 350.degree. F. to room temperature is accomplished at a relatively low quench rate, using air as the quenching medium. The quenched product is then aged to obtain a substantially stable level of mechanical properties. The automotive material so prepared is shown to exhibit stable strength for many months while exhibiting good formability.
Alloys of the Aluminum Association ("AA") 6000 series present an additional problem. The natural aging of AA 6000 series alloys at room temperature is detrimental to the artificial aging process. The clusters formed during natural aging are too small. Although these small clusters grow larger during prolonged room temperature aging, they still fail to achieve the critical size that is necessary to be stable at the subsequent artificial aging temperature. Furthermore, the growth of these dusters during natural aging also depletes the supersaturation of solutes in the matrix. These two mechanisms hinder the precipitation of Mg.sub.2 Si during the artificial aging. As a consequence, artificial aging response is reduced by prolonged room temperature aging resulting in diminished mechanical properties of the AA 6000 alloy after artificial aging.
Ideally, after forming, the sheet would receive a separate aging treatment to increase its strength to the maximum possible for the particular AA 6000 series alloy. The economies of automobile production, however, require that the metal strength be increased by the baking used to harden the paint on the partially assembled vehicle. Unfortunately, the paint baking temperatures are lower than the optimum, and it is necessary to modify the sheet properties to increase strength levels achieved in the paint bake.
Workers in the field have made several attempts to process AA 6000 series alloys having improved bake hardenability or paint bake response. For example, U.S. Pat. No. 4,718,948, to Komatsubara et al., describes a rolled aluminum alloy sheet of good formability for automotive purposes. The sheet products made from a AA 6000 series aluminum alloy containing a relatively high quantity of silicon, from 1.25 to 2.5 wt.%. The sheet product is first subjected to a solution heat treatment at about 1000.degree. F. and then quenched to room temperature at the rate of about 1800.degree. F./min. The quenched sheets are then aged at room temperature for about two weeks and the aged sheet products are claimed to have improved mechanical and forming properties, particularly improved baize hardenability.
Similarly, U.S. Pat. No. 4,808,247, also to Komatsubara et al., describes a AA 6000 series aluminum alloy rolled sheet of improved formability and yield strength. These improved properties can, according to the reference, be obtained by solution heat treating the sheet made from the aluminum alloy for at least 5 seconds at temperatures between 500.degree.-580.degree. C., followed by rapid quenching to room temperature at a rate within the range of 5.degree.-300.degree. C./sec. If good sheet flatness is not a consideration and only high strength is desired quenching rates in excess of 300.degree. C./sec. are recommended by the patent.
Likewise, U.S. Pat. No. 4,897,124, to Matsuo et al., concerns a AA 6000 series (Aluminum-Silicon-Magnesium) aluminum alloy rolled sheet exhibiting improved properties, such as good formability, elongation, high strength and corrosion-resistance. When the aluminum alloy sheet of this reference is utilized for automotive body sheets, such body sheets possess improved post-bake strength. To obtain these improved properties the rolled sheet is subjected to a solution heat treatment at 450.degree.-590.degree. C., followed by rapid quenching to room temperature at a rate of not less than 5.degree. C./sec.
In general, the above references describe solution heat treatment followed by a rapid quench to room temperature to obtain improved paint bake response. In U.S. Pat. No. 5,266,130, to Uchida et al., a two-stage quenching process is used to produce a AA 6000 series rolled aluminum alloy sheet having improved shape fixability and bake hardenability. Specifically, these improved properties are obtained by solution heat treatment of the rolled sheet at 450.degree.-580.degree. C., followed by a two-stage quenching process. In the first stage of the quenching process, the solution heat treated sheet is cooled to a temperature within 60.degree.-250.degree. C. at a rate of 200.degree. C./min. or more, followed by a second stage cooling to a final temperature of 50.degree. C. at a significantly lower cooling rate. The reference teaches that the cooling rate of the second stage quench must be done at a slow rate to prevent the formation of GP zones which result in poor bake hardenability. The main drawback of this reference is that the sheet cannot be allowed to cool below 50.degree. C. (122.degree. F.). Cooling below 122.degree. F. would probably result in the formation of GP zones and poor bake hardenability. This is a serious practical limitation because the operation of commercial plants require that sheet material be held for several hours at room temperature before it can be further processed.
In general, aluminum sheet is processed as coils and involves many steps, including hot rolling, cold rolling, trimming, annealing, heat treating, quenching, and leveling. For economical processing, it is fed from process to process as strip in a continuous manner. The continuous nature of the process puts constraints on the individual processes which must be adjusted to fit the speed of the strip, which in turn is strongly governed by the economics of the total process.
Although the overall processing is constrained by the speed of the strip, the individual steps of the process can be controlled in a number of ways, including by adjusting the temperature, and by choosing the length of the path through the process equipment, which in fact controls the time in which the aluminum is in the individual process.
The time in each process is, however, limited by certain practicalities such as the necessity to use existing equipment, and limitations on the length of the process equipment. Thus, a need remains for a process which uses existing heat treating equipment without forcing the production line to be operated at non-economically slow speeds. A solution to these limitations can be to remove the aluminum alloy from the continuous process and perform a batch process. Batch processing of the aluminum sheet would, however, require that the sheet be held or stored, generally at room temperature, for several hours or even up to a day before it can be further processed.
Thus, a need remains for a method of producing AA 6000 series aluminum alloy rolled sheet that exhibits improved formability and improved strength after low temperature aging as caused by the paint baking step used in the curing of paint on new automobiles and yet can be stored at room temperature for up to a day before further processing without having significantly diminished physical properties after the paint bake step. Accordingly, it is an object of this invention to provide such a method.